In automatic transmissions, closed-throttle upshifting is performed when the driver releases the accelerator pedal in a mid to high-speed driving state (closed-throttle downshifting occurs as the vehicle slows down on closed throttle). In the prior art, duty is controlled in two stages during closed-throttle upshifting. This can be illustrated as two distinct "ramps" on a graph.
However, shifting is not realized at predetermined gear meshing points nor at a point where the duty control value is "0" as a result of various mechanical and hydraulic pressure control errors. As a result, shift shock occurs by the difference in engine RPM and turbine RPM.
Further, it is possible for hydraulic pressure, operating to perform the upshifting, to be excessively raised such that shifting is forcefully realized before the predetermined gear meshing points. This also results in shift shock caused by the difference in engine RPM and turbine RPM.